Anal. Chem. 2009, 81, 8185–8192
Rapid Determination of Plutonium Isotopes in Environmental Samples Using Sequential Injection Extraction Chromatography and Detection by Inductively Coupled Plasma Mass Spectrometry Jixin Qiao,† Xiaolin Hou,*,† Per Roos,† and Manuel Miro´‡ Radiation Research Division, Risø National Laboratory for Sustainable Energy, NUK-202, Technical University of Denmark, DK-4000 Roskilde, Denmark, and Department of Chemistry, Faculty of Sciences, University of the Balearic Islands, Palma de Mallorca, Spain This article presents an automated method for the rapid determination of 239Pu and 240Pu in various environmental samples. The analytical method involves the inline separation of Pu isotopes using extraction chromatography (TEVA) implemented in a sequential injection (SI) network followed by detection of isolated analytes with inductively coupled plasma mass spectrometry (ICP-MS). The method has been devised for the determination of Pu isotopes at environmentally relevant concentrations, whereby it has been successfully applied to the analyses of large volumes/amounts of samples, for example, 100-200 g of soil and sediment, 20 g of seaweed, and 200 L of seawater following analyte preconcentration. The investigation of the separation capability of the assembled SI system revealed that up to 200 g of soil or sediment can be treated using a column containing about 0.70 g of TEVA resin. The analytical results of Pu isotopes in the reference materials showed good agreement with the certified or reference values at the 0.05 significance level. Chemical yields of Pu ranged from 80 to 105%, and the decontamination factors for uranium, thorium, mercury and lead were all above 104. The duration of the in-line extraction chromatographic run was 104 for 232Th was obtained when using 50-60 mL of solution composed of 6 or 9 mol/L HCl following the HNO3 washing (see groups 8-11 in Table S-4). Finally, a 9 mol/L HCl solution was adopted for the removal of Th, which is in good agreement with earlier findings.34,44,45 Flow rates for sample cleanup were varied within the range of 1.0 to 5.0 mL/min (see groups 7, 8, and 12 in Table 2) aimed at increasing the sample throughput and, thus, improving the analytical efficiency without significant deterioration of the separation efficiency and the chemical yield for target radionuclides. At 5.0 mL/min the chemical yield of Pu and the decontamination factors for U, Th, Hg, and Pb are still acceptable, yet, because of the increase of the backpressure observed in the SI-column system, the washing flow rate was affixed to 2.5 mL/min for the remainder of the work. The sample loading flow rate was however set to 1.0 mL/min to ensure quantitative uptake of Pu as demonstrated by the results of chemical yields of 242Pu compiled in Table S-4. Elution of Plutonium. A variety of eluents for the elution of Pu from TEVA resin have been reported including diluted HF-HNO3 solution,7,8 diluted HF-HCl solution,43 0.5 mol/L HCl,6 0.1 mol/L H2C2O4-0.05 mol/L HNO3,45 0.01 mol/L (NH4)2C2O4,24 and 0.1% hydroxylethylidene diphosphonic acid.9 Earlier research-
ers demonstrated that Pu can be efficiently eluted from TEVA resin by a reducing solution (e.g., hydroxylamine hydrochloride) in diluted acid media as a result of the reduction of Pu to Pu(III).46,47 Because of the eventual concomitant presence of residual amounts of Pb and Hg onto the column, and the formation of polyatomic molecules, such as 204Pb35Cl, 204Hg35Cl, and 202 Hg37Cl in a HCl medium, an eluent composed of 0.1 mol/L NH2OH · HCl in 0.1 mol/L HNO3 was assessed as an eluting medium for Pu to facilite direct injection into ICP-MS. However, experimental results (see groups 2-4 in Table S-4) indicates distinctly lower decontamination factors for 238U (90%. Yet, direct injection of the eluate into ICP-MS rendered 2-fold decreased efficiency for Pu with respect to that of a standard solution in 0.5 mol/L HNO3 medium (see Table S-5 for further details) as a consequence of the high concentration of chloride ions in the eluate. Therefore, wherever off-line detection was conducted, the eluate was first evaporated to dryness and reconstituted in 0.5 mol/L HNO3 as detailed in the Experimental Section to improve the detection sensitivity and reduce the risk of generation of polyatomic interfering species. Reusability of TEVA Resin and Carryover Effects. The potential reusability of the TEVA resin is crucial for the development of a fully automated and cost-effective SI-extraction chro-
(44) Lee, S. H.; La Rosa, J.; Gastaud, J.; Povinec, P. P. J. Radioanal. Nucl. Chem. 2005, 263, 419–25. (45) Dewberry, R. A.; Boyce, W. T. J. Radioanal. Nucl. Chem. 1999, 240, 643– 50.
(46) Chen, Q. J.; Dahlgaard, H.; Nielsen, S. P.; Aarkrog, A. J. Radioanal. Nucl. Chem. 2002, 253, 451–58. (47) Donard, O. F. X.; Bruneau, F.; Moldovan, M.; Garraud, H.; Epov, V. N.; Boust, D. Anal. Chim. Acta 2007, 587, 170–79.
Analytical Chemistry, Vol. 81, No. 19, October 1, 2009
8189
Table 1. Evaluation of the Analytical Performance of a Single TEVA Column Utilized in Repetitive Assaysa 239
Pu measured
sample number 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 average(±SD)
addition of
239
Pu (mBq)
6.13 0.00 23.07 5.84 0.00 21.04 5.36 0.00 20.43 4.87 0.00 23.52 5.23 0.00 20.48 0.00 22.28 5.56 20.27 5.71
chemical yield (242Pu; %) 95.4 99.8 100.0 100.0 95.8 104.3 100.0 100.0 104.6 100.0 100.0 100.0 100.0 100.5 103.9 100.0 100.0 98.1 100.0 94.8 99.9 ±2.6
(mBq) 8.01 1.40 23.06 7.50 1.68 22.22 6.78 1.47 22.86 5.96 1.42 23.43 6.55 1.44 22.11 1.44 23.72 6.79 18.74 7.43
decontamination factor
(%)b
238
106.4 100.0 94.2 103.6 120.0 99.0 100.3 105.0 104.7 95.1 101.4 94.0 98.8 102.9 101.1 102.9 100.2 97.6 86.5 104.5
× × × × × × × × × × × × × × × × × × × × ×
232
U
5.7 2.2 1.6 1.9 2.6 9.5 3.4 1.0 7.2 1.2 1.2 1.3 1.7 1.6 4.0 2.1 4.3 3.0 1.9 6.5 6.4
208
Th
4
10 104 104 104 104 103 104 105 104 105 105 105 105 105 104 105 104 105 104 104 104
2.1 2.9 2.5 2.4 2.2 1.5 2.1 2.3 2.1 1.3 5.8 1.0 1.0 4.9 8.8 7.3 6.7 1.2 2.2 1.7 1.6
× × × × × × × × × × × × × × × × × × × × ×
Pb
4
10 104 104 104 104 104 104 104 104 104 103 104 104 103 103 103 103 104 103 103 104
1.4 4.2 2.7 5.2 3.1 4.9 1.3 3.8 3.6 1.8 3.3 2.8 2.6 6.7 1.2 1.7 5.0 2.9 4.1 3.0 2.0
× × × × × × × × × × × × × × × × × × × × ×
104 104 104 103 104 103 104 104 104 104 104 104 104 103 104 104 103 104 103 103 104
a The reported 239Pu concentration in the Danish soil sample is 0.140 ± 0.008 mBq/g, the total 239Pu in 10 g of sample is 1.40 ± 0.08 mBq. The uncertainties of the chemical yield measured by 242Pu and 239Pu activity are
